Brushless permanent magnet motors are well known to have significantly higher torque-to-copper loss ratios then other kinds of motors, such as induction motors and switched reluctance motors. Copper loss values vary as a square function of total applied motor current, and are a primary factor in determining motor temperatures at standstill and at low speeds.
The torque output of a permanent magnet motor is substantively higher than that which can be produced by a non-permanent magnet motor of the same size, operating at the same temperature. This is so because the copper losses that are associated with the external electrical power used to provide the magnetic bias in a non-permanent magnet motor do not occur in permanent magnet motors. Moreover, currently available permanent magnet materials have very high magnetic energy densities, making it feasible to provide motors of comparable magnetic field strength, in which the electromotive driving force is generated using external electrical power and coils, only if active external cooling is applied.
It is to be appreciated that primary advantages afforded by permanent magnet motors at standstill and low speeds are generally regarded to become disadvantages at medium and high speeds. This is attributable to a number of factors, beyond those which are inherent in the high magnetic energy density of the magnets used. One such factor concerns the magnetic response characteristics of electrical steel. Iron losses vary as approximately square functions of frequency and flux, and at high speeds the flux densities that are magnetically induced in the stator (i.e., in the d-axis poles) cause iron losses to vary as a cubic function in the affected areas; thus, significantly increased amounts of power are consumed and commensurately elevated motor temperatures are generated. In addition, the rapid changes that occur in flux density generate correspondingly high values of back electromotive force (b-emf), thus further increasing the motor supply voltage demand, especially for the attainment of high speeds.
The inherent characteristics of permanent magnet motors are of course reflected in the kinds of applications for which they are typically employed. For example, permanent magnet motors dominate in positioning servo systems for industrial automation, in which applications the motor operates primarily to regulate about a selected position or to move from one position to the next; for such purposes high torque at standstill and lower speeds, and relatively small physical size, are primary criteria. Conversely, periods of high-speed operation in such applications are very brief, and the accompanying energy losses are accepted; moreover, high-voltage supplies are of course available, as needed, on a factory floor.
On the other hand, in mobile applications, such as for hybrid and electric vehicles (e.g., automobiles, buses, trucks, motorcycles, scooters, mopeds and, indeed, the so-called human transporters that have recently been introduced), the motor used must be capable of operating at high speeds and for long periods of time, and low efficiency at operating speeds is not acceptable. The difficulty of satisfying these criteria is increased by the need to use (now, and for the foreseeable future) low-voltage batteries, which severely limit speed range capability. As a result of such factors, induction motors and (to a lesser extent) switched reluctance motors are presently employed for vehicular applications, with the low torque that they produce during acceleration being either accepted or compensated for, typically (as to the latter) by overdriving of the motor; electrical power consumption and motor temperatures are therefore excessive, and battery life is significantly reduced.
Because of the recognized inherent benefits of permanent magnet motors, attempts have been made to overcome their disadvantageous high-speed characteristics. These efforts have usually employed so-called “field weakening” techniques, which are well known as a means for changing output characteristics of brush-type motors. The prior art indicates that analogous or equivalent measures have been attempted for brushless permanent magnet motors, using d-q control systems, tapped phase windings, and mechanical approaches.
More particularly, in accordance with U.S. Pat. Nos. 6,304,052, 6,057,622, and 6,008,614, d-q control is used in so-called “internal permanent magnet motors.” They employ either steel bridges disposed across the magnets (inserted magnets) or between stator poles (magnetic short circuit bridges), or very thin permanent magnets. Proponents assert that the application of d-axis phase excitation increases high-speed performance of such motors; this could occur, however, only at the expense of significantly reduced low-speed torque capability.
Although the foregoing effect may be ameliorated by using narrower bridges, still low-speed torque values would typically be reduced to less then half of those which could be produced using standard surface magnet rotors, and the application of d-axis current would be expected to have limited range and effectiveness. This is so because b-emf is zero in the d-axis and highest in the q-axis, and the application of d-axis current could therefore affect b-emf only as a minor second order effect.
On the other hand, flux density is zero in the q-axis and has maximum values in the d-axis, which latter factor causes iron losses to approach a cubic function in the poles. The application of d-axis current (opposing the permanent magnet flux) reduces flux density in the d-axis poles, causing cubic (iron) losses to be replaced by square (copper) losses; the difference represents energy that becomes available for producing higher output speeds.
The stronger effect (resulting from the use of wide bridges) would be to even more greatly reduce low speed torque, causing it to approach the level of induction or synchronous reluctance motors but allowing the range of effect using d-current to be extended somewhat. The reduced flux density in the stator poles, caused by the use of wide bridges, would render the above-described cubic-to-square loss exchange less dominant, possibly making it only a second-order effect while, at the same time, possibly increasing the significance of the previously mentioned second-order effect. The influence component (d-axis current) has twice the electrical frequency of the permanent magnetic field, and even partial recovery of its generated torque is cumbersome, at best.
The mechanisms described above show the limitations of the d-q approach, the output range being restricted because of the loss-exchange nature of d-q control. For the same reason, motor temperatures at high speeds remain high, and the volume of active magnets, and low-speed torque values, are correspondingly low. Thus, the primary advantage and attractiveness of the permanent magnet motor (i.e., low speed torque) is significantly compromised.
In accordance with the technique disclosed in U.S. Pat. No. 5,811,905, the phase windings of the motor described are electrically tapped at intermediate points; the full windings are utilized at low speeds, and only portions of the windings (e.g., half, assuming mid-point taps) are employed at higher speeds. This technique produces an extended speed range by reducing the b-emf to a proportion of the value it would otherwise have had (e.g., to half). It does not however alter the magnet bias field, and iron losses remain very high in the d-axis poles, again resulting in reduced efficiency and high motor temperatures. In addition, difficulties are associated with the switching that is involved in alternating between full and partial energization of the phase windings.
Masuzawa et al. U.S. Pat. No. 5,821,710 and Rao U.S. Pat. No. 6,194,802 disclose motors in which centrifugally operated mechanical means is used for producing magnetic field weakening effects. More specifically, the Rao patent describes a motor in which magnets, supported in a carrier on the rotor, move radially outwardly in response to the centrifugal forces that are generated by rotor rotation. This technique reduces magnet-to-stator engagement and, in turn, decreases flux linkage and b-emf, ultimately leading to higher speeds; difficulties may arise however due to several factors: Not only are the radial forces acting upon the magnets a function of motion length, and hence highly nonlinear, but also, and perhaps more importantly, the magnitudes of those forces necessarily vary significantly with the speed of rotor rotation. There would also appear to be substantial differences in the radial forces that occur between d and q axis magnets, due in part to the current return path in the stator winding. Such force variations would be expected to induce oscillatory motion in the magnets during operation of the motor, making it difficult, if not impossible, to map magnet actuation into a useful speed range.
Masuzawa et al. employ a rotor that is comprised of two, axially adjacent bodies or sections, one being rigidly attached to the rotor shaft and the other being mounted for limited angular displacement relative to the first. When the rotor sections are normally aligned (i.e., not angularly displaced relative to one another) magnets of the same polarity on both sections are in mutual registration, and b-emf and torque are both maximized. Displacing the sections an angular distance corresponding to one magnet pitch (i.e., bringing N and S poles on the two sections into mutual registration) causes b-emf to be reduced to a very low value. Magnetic flux travels axially along the stator, but does not penetrate to a depth sufficient to enter the coils.
Measurements made using a prototype similar to the motor described in the Masuzawa et al. patent, but differing therefrom in that magnet volume and surface area were higher, produced the following results: Even at speeds as low as 2500 rpm, and flux densities common in modern conventional motors, difficulties were found to exist in the magnetic concept; i.e., as the rotor sections were angularly displaced b-emf decreased and measured iron losses increased (rather than decreasing). At speeds higher than 2500 rpm, moreover, iron losses became so dominant that efficient motor operation was not possible, evidently due to the generation of axial flux in the stator poles (axial flux polarity alternates according to electrical frequency, and flux density is high). In addition, due to the orientation of the planes of the stator laminations, large eddy current losses were induced in the stator pole tips, thus severely compromising high-speed efficiency. And finally (and although not tested), control difficulties would be anticipated in the use of such a motor; because the angularly shiftable rotor section is not constrained, interaction between the two rotor sections would be expected to transmit unanticipated and unpredictable torques to the motor load.